Grafting, fused corpses, and human experimentation appears a lot in Elden Ring, Rykard literally becomes part of a serpent, Miriel talks about how all things can be conjoined. Miquella's unalloyed gold, is contrasted to the Golden Order which conjoins other belief systems. The Fire Monks worship of the Fell God and the Worship/study of the Blood Star were both normalized parts of Erdtree society, not to mention the Dragon Cult as well and the Golden Order itself took in Lucarian ideals when Radagon married Marika.
Radagon being Marika also has nothing to do with the malleable flesh of Numen, we're shown a 1:1 parallel with Miquella and Saint Trina, Radagon consists of the parts of herself that Marika deemed ungodly and abandoned and then they re-merged when Radagon married Marika and implemented the Law of Regression.